The present invention relates to a method and a circuit arrangement for measuring a current according to magnitude and phase angle and can be used, for example, to precisely trigger electronic tripping devices in circuit breakers.
Precise compliance with specified tripping characteristics and display of current values is now required for electronic tripping devices in circuit breakers. One conventional method of increasing current measurement precision is to use Rogowski coils with a downstream integrator and normalizing amplifier.
A further conventional method of enhancing precision is to increase the sampling rate of the analog signal and improve the software analysis of the sampled values.
A method for sampling measured quantities and further processing the sampled measured quantities to generate a tripping signal as a function of the magnitude and duration of an overcurrent is described in German Patent No. 35 22 739. According to this method, the sampled measured values are converted to digital values that are weighted and added up according to a specified function. The tripping signal is formed when a cumulative value lies above a specified reference value.
Increasing the sampling rate is a common conventional method. However, this method requires powerful processors that are relatively expensive and consume a great deal of power. The price and power consumption thus limit the ability to increase the sampling rate for economic and technical reasons.
Improving precision through software analysis is based on the fact that a sinusoidal valuexe2x80x94the fundamental wave in this is uniquely determined by a pair of values, namely the magnitude and angle. Thus, one sampled value per period would be entirely sufficient to determine the current with an adequate degree of precision in currents without harmonics.
However, these idealized conditions do not exist in practice. Instead, the practical reality is that harmonics do occur in operating currents, and there is the difficulty of precisely determining the phase angle in harmonic currents. For these reasons, current measurement or analysis of this type has not been previously used.
Previously, the root mean square was calculated by measuring the largest possible number of sampled values per period without taking the phase angle into account. The sampled values were squared, added up over a specified period of time, and the average thus formed. An electronic tripping unit for a circuit-breaker, in which a microcomputer is used to determine the root mean squate of the conductor current, is described in German Patent No. 43 23 356.
An object of the present invention is to provide a method and a circuit arrangement for measuring a current, using simple means to accurately measure the current and enabling precise, reliably, and reproducible tripping of circuit breakers.
One particular advantage of the present invention is that the tripping precision of electronic tripping devices in circuit breakers can be significantly improved at a low cost, using simple means, by measuring the current according to magnitude and phase so that only the fundamental wave of the current is sampled at a low sampling rate between precisely determined current zero crossings, and the root mean square is calculated on the basis of the measured phase angle and the amplitude. To achieve this, a PLL circuit with a downstream zero switch is provided parallel to the input of the A/D converter and to a port input of the microprocessor.